The phenomenon of photobleaching of a photosensitizer during photodynamic therapy (PDT) is well known. For second generation photosensitizers it may be possible to exploit this effect to enhance the volume of damaged tissue and improve the efficacy of PDT. In addition, as a consequence of photobleaching, the fluorescence emitted by the photosensitizer will decrease during PDT. A diffusion theory model has been developed which simulates photobleaching of a photosensitizer during PDT and calculates the total fluorescence emission as a function of delivered light fluence. The fluorescence signal can be compared with tissue necrosis boundaries calculated for realistic treatment conditions using a threshold model previously described by us. The relationship between the fluorescence signal and the extent of tissue damage is discussed.